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Water: Reclamation And Conservation Applications

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Reusing water in a building can significantly decrease operating costs of any building. There are currently many systems that can save money and protect the environment at the same time. One of these systems is to harvest rainwater through rooftop drains and collecting it into a cistern to then be used for numerous applications. Another means of collecting water for reuse is by capturing the water that is produced in the dehumidification process of a buildings air management system. One way that anyone can save on water consumption is to use products that are WaterSense (an EPA organization similar to Energy Star) labeled, these products significantly reduce the amount of water used through toilets, faucets, sprinklers, etc. All these systems and products lower the amount of water used in a building therefore reducing utility costs and the amount of water pollution.

One way to decrease utility costs and the amount of polluted water is to harvest rainwater. Initially harvesting rainwater was done by setting empty barrels and buckets outside of a home. These buckets and tanks were used mainly for watering gardens and plants. Today these primitive systems have been developed for much larger and different applications, as well as to earn LEED points. One major difference is in the way these are being applied, they are now being used to provide water for use in toilets mainly. The type of building and location play a big role in the type of application. For example, low to mid-size commercial buildings would be prime candidates to use collected water for toilets due to their rooftop area to toilet ratio. A good example of using water collected for irrigation applications would benefit mostly by warehouse type buildings since they have a greater rooftop area and few toilets to flush. One other application in which collected water can be used is as a supply for fire protection sprinkler water as it might be used in high-rise building and upsizing the tank, for example (Goedken, Rainwater-Harvesting).

The way that rooftop water harvesting systems work is by using the rooftop as the collection point for rainwater. The roofs of buildings using these systems slope slightly and have a rooftop drain at the center and lowest point of the sloped area. Typically there will be a few drains per building and will be located along some central line and be evenly spaced in order to provide maximum efficiency and lowest chance of backup in the drainage system, the larger the area the more amount of drains needed. The drains themselves that are used will vary in size depending on the surface area in which they are collecting for and the average annual rainfall in that location, the larger the bigger in size. A roof washer is installed to remove dirt, leaves, and debris from the roof periodically, ensuring clean water.

The water is then transported as usual through downspouts but instead of the water being emptied as ground water or into the sewer it is collected in a big storage tank. This "big storage tank", which is known as a cistern, is typically located underground in parking garages or basements. These cisterns are made of "epoxy steel, fiber glass, precast concrete, polyethylene, or poured-in-place concrete" ( Goedken, Rainwater-Harvesting). Similar to the size of roof drains the size of the tank depends on the average annual rainfall in that location and the size of the roof, the other determinant is how much water is to be collected at any time in order to efficiently provide water to the system/application that is using the water. For example you would only need a small tank if you are only collecting water to operate a couple bathrooms in a building that doesn't have a high demand on those bathrooms, and if that building is located somewhere with a high average annual rainfall. The reason being is that if there isn't a heavy demand for water and if that water can be replenished quickly by a high average rainfall then a smaller tank can operate the system well.

The water that is collected in the tank is then either used immediately, like for use in irrigation and fire protection or it will be treated for various uses, like in bathrooms for use in toilets and sinks. The water that is returned from use from bathrooms and other drains, known as black water, will then get treated and used again if possible. Below is a diagram showing a similar type of system (Goedken, Rainwater-Harvesting).

This system collects rooftop rainwater via rooftop drains. The water is then collected in a cistern which they call "Fire Water Storage". The water is then used to remove heat in a cooling tower. From there the water is treated, indicated by the green line and box labeled "Blackwater Treatment". The water, now described as "Non-Potable Water" is used by a bathroom located in the building. The used water from the bathroom, indicated by the grey line and now described as "Blackwater Sewer" travels back to get treated as it did initially is indicated by "Blackwater Treatment", at this point water that cannot be treated exits the system. The water that can be used will be used again in the bathroom, by the cooling tower, or be collected in the cistern, described as before as "Fire Water Storage", via infiltrated groundwater or direct plumbing. From there the cycle begins again for that water and new water is introduced as before by collecting rainwater from the roof.

There are some issues with using a rainwater harvesting system. First of all this type of system is most economical to new construction since it requires dual plumbing. Another issue is relying on rain to supply a system with its water since you can never determine the amount of rain and how often it will occur. For example if it doesn't rain for a long time then their might not be enough water stored in the cistern, therefore no water to use as desired. If the cistern was bigger then it could handle a longer period of time without rain. But then this would increase the cost of the system, making the system cost inefficient.

The basic mathematical principle for figuring out the size of the cistern is that "for every inch of annual rainfall, it's possible to collect 600 gallons of water per 1,000 square feet of roof area. A good way to estimate the cost of a rainwater harvesting system in new construction is that for every one gallon of capacity of the cistern the cost increases a dollar" (Goedken, Rainwater-Harvesting). These two relations of needed capacity and cost show how the smallest cistern that you require is the best due to price. Having a small cistern can cause problems with demand that is why other systems are used to supplement rainwater harvesting like collecting water through the dehumidification process.




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